A conventional window covering system includes a headrail, a bottom rail, a covering material, and a driving module. The covering material is mounted between the headrail and the bottom rail. The driving module is mounted in the headrail, wherein the driving module can operate with the covering material simultaneously such that the covering material is collected or expanded. The driving module includes a spiral spring, wherein the spiral spring includes a first end and a second end. The spiral spring winds toward the first end or the second end while the covering material is collected or expanded, whereby a restoring force is generated. The restoring force of the spiral spring is less than the weight force from the bottom rail, therefore the bottom rail descends to expand the covering material when a restriction of the bottom rail is removed, thereby achieving light blockage. The bottom rail ascends when an upward force applied by a user and the restoring force from the spiral spring are acting upon the bottom rail, whereby the covering material is collected for removing the light blockage.
However, the covering material is rapidly expanded because of the weight force from the bottom rail when the restriction of the bottom rail is removed, which causes damages to the window covering system due to collision with the surroundings. In addition, the spiral spring winds rapidly with the rapid expansion of the covering material, which results in elastic fatigue of the spiral spring, and thus insufficient restoring force can be provided. Although it is possible to avoid rapid expansion of the covering material and elastic fatigue of the spiral spring by using an operating cord to control an expansion speed of the covering material, using the operating cord is time consuming and inconvenient, therefore the window covering system should be improved. Furthermore, the elastic fatigue of the spiral spring causes undesired descending of the bottom rail after the covering material is collected, such that the covering material can not be completely collected. Some people uses the spiral spring with a larger restoring force to overcome the elastic fatigue problem, but the user must apply a greater downward force to resist the larger restoring force so as to remove the restriction of the bottom rail, thus inconvenience of operation.